This invention relates to a method and apparatus for determining the concentration of one or more than one gaseous component, especially a carbon monoxide component, in a mixture of gases by the use of metallic oxide semiconductors the electrical resistances of which change in the presence of reducing gases.
A well-known method for determining the carbon monoxide concentration in a mixture of gases is based on the selective infra-red absorption of the gaseous mixture through which infra-red radiation is passed. This method is specific for the gaseous components to be measured and yields very reliable results. The measuring instruments used for carrying out this method have proved technically satisfactory but, because of the necessary optical and mechanical devices required for measuring the absorption of the infra-red radiation, they are complicated and expensive. Moreover, these measuring instruments are relatively highly sensitive to moisture and they are also sensitive to shocks.
Monitoring instruments are also known in which metallic oxide semiconductors, such as stannic oxide, zinc oxide or ferric oxide, are used as measurement indicators, the electrical conductivity of these semiconductors increasing in the presence of reducing gases such as hydrogen, carbon monoxide or methane. These metallic oxide semiconductors are very sensitive since in the presence of reducing gases their electrical conductivities, and similarly their electrical resistances, change by several decimal powers, so that these metallic oxide semiconductors can be used as switches. A drawback is that the metallic oxide semiconductors respond to a greater or lesser extent to all reducing gases, so that a measuring instrument equipped with these metallic oxide semiconductors can only be used for determining the sum of the combustible gaseous components in a mixture of gases. Added to this, the metallic oxide semiconductors are unstable, for in the course of time gas molecules diffuse into the surface of the semiconductor, causing the sensitivity of the semiconductor to change in an uncontrollable manner.
As, moreover, the sensitivity of these metallic oxide semiconductors is dependent to a marked degree on the effective duration of the presence of the reducing gaseous components and the temperature, measuring instruments making use of metallic oxide semiconductors for determining the content of one or more than one gaseous component in a mixture of gases, especially in mining, are not yet available. In underground mineworking operations, an unambiguous reading, for example of the carbon monoxide content in the range of from 0 to 50 p.p.m., is required from a measuring instrument for determining the carbon monoxide concentration in the gas mixture, and the cross-sensitivity in respect of methane should be less than 5 p.p.m. carbon monoxide with a methane content of 1%.